Owing to improved storage capacity, more frequent rechargeability and increased energy densities, batteries for example batteries based on lithium-ion cells or nickel metal hydride batteries, are used in ever broader applications. Batteries having a lower energy storage capacity are used, for example, for small portable electronic appliances such as mobile telephones, laptops, camcorders or the like, while batteries with a high capacity are used as energy source for the drive of motors of hybrid or electric vehicles, etc. or as stationary batteries.
Batteries can be formed, for example, by interconnecting battery modules in series, wherein there are sometimes also parallel interconnections of the battery modules, and the battery modules for their part can also consist of battery cells connected in series and/or parallel.
For driving motors of hybrid or electric vehicles, in particular polyphase battery systems are suitable, which are also referred to as battery direct inverters (BDIs) and are described in DE 10 2010 027 864. BDIs comprise battery modules and coupling units assigned to the modules. The battery modules are individually electrically connectable and electrically disconnectable by means of the coupling units, optionally with the possibility of choosing between a positive and a negative connection.
FIG. 1 shows a battery module 60 comprising a coupling unit 50. A plurality of battery cells 11 is connected in series between the inputs of a coupling unit 50. This battery module 60 is not restricted to such a series circuit of battery cells 11; only a single battery cell 11 can also be provided, or else a parallel circuit or mixed series-parallel circuit of battery cells 11. The first output of the coupling unit 50 is connected to a first terminal 61, and the second output of the coupling unit 40 is connected to a second terminal 62. The battery cells 11 can be decoupled from the rest of the battery on both sides by the coupling unit 50, which enables hazard-free replacement during running operation, for example, since the hazardous high summation voltage of the rest of the battery modules of the battery is not present at any pole of the battery cells 11.
FIG. 2 shows a battery which has n battery module strings 70-1 to 70-m. Each battery module string 70-1 to 70-m has a plurality of battery modules 60, wherein preferably each battery module string 70-1 to 70-m has the same number of battery modules 60 and each battery module 60 has the same number of battery cells 11 interconnected identically. One pole of each battery module string 70-1 to 70-m can be connected to a corresponding pole of the other battery module strings 70-1 to 70-m, which is indicated by a dashed line in FIG. 2. In general, a battery module string 70-1 to 70-m can contain any number of battery modules 60 greater than 1 and a battery can contain any number of battery module strings 70-1 to 70-m. In addition, charging and disconnecting devices and disconnecting devices can also be provided at the poles of the battery module strings 70-1 to 70-m when safety requirements demand this. However, such disconnecting devices are not necessary because decoupling of the battery cells 11 from the battery connections can take place by the coupling units 30 or 50 contained in the battery modules 60.
FIG. 3 shows a drive system comprising a battery. In the example shown, the battery has three battery module strings 70-1, 70-2 and 70-3, which are each connected directly to an input of a drive motor 13 designed for operation with three phase signals. By virtue of a control unit of the battery activating (or deactivating) a variable number of battery modules 60 in a battery module string, a voltage which is proportional to the number of activated battery modules 60 and which may be between 0 V and the total output voltage of the battery module string is made available at the output of the battery module string.
Therefore, the polyphase battery system can be used by corresponding connection and disconnection of the modules to generate a plurality of AC voltages which are phase-shifted with respect to one another. Given a corresponding design, for example, three AC voltage profiles which approximate sinusoidal curves and are phase-shifted with respect to one another can be generated, as a result of which a rotating field which can be used directly for driving electric or hybrid motors is generated. The voltage is in this case selected such that the alternating current which is set brings about the torque which is required for operating the motor.
FIG. 4 shows a profile over time of an output voltage of a battery module string. The output voltage of one battery module string V is in this case plotted over time t. An (ideal) sine wave which is desired for an exemplary intended application but which only has voltage values greater than or equal to zero is plotted using the reference symbol 80-b. The ideal sine wave is generated approximately by the battery module string by a discrete-value voltage curve 80-a. The discrepancies between the discrete-value voltage curve 80-a and the ideal curve 80-b are dependent in terms of size on the number of battery cells 11 which are connected in series in a battery module 60. The fewer battery cells 11 are connected in series in a battery module 60, the more precisely the discrete-value voltage curve 80-a can follow the idealized curve 80-b. In customary applications, the comparatively small discrepancies do not impair the operation of the entire system, however.
Battery management systems are used for battery management, for example for basic actuation of modules, for increasing the safety of batteries, for increasing efficiency and for extending the life of battery modules and battery systems comprising battery modules. One function of battery management systems is to record current intensity and/or voltage profiles over time with an accuracy and sampling frequency that is necessary for determining a present state of charge and/or a present state of ageing, an internal impedance, a temperature value and/or the value of another characteristic variable of the individual battery modules.
Although the production of the battery cells is standardized and excellent production methods are implemented by the cell manufacturer, differences in the electrical properties, for example with respect to voltage, capacitance or internal resistance, are observed. In addition to these differences which arise owing to the production process, ageing processes cause a further change in the electrical properties of the battery cells. In a battery module comprising one or more cells, the individual battery cells are therefore never identical in respect of the electrical power. In particular in the case of a series interconnection of the cells in a module, the response of the module is determined by the electrical characteristic of the weakest cell. Therefore, the individual battery modules likewise have individual electrical characteristics.